1. Field of the Invention
This invention relates to an encoding circuit for serial transmission of binary data through an information channel. More particularly, it relates to a digital modulation system which is effective in high density data recording or the like.
2. Related Art Statement
A so-called D-2 format, currently employed in a composite digital video tape recorder, makes use of an M.sup.2 code (Modified Miller Code) as a code system for digital modulation. The M.sup.2 modulation code is a non-block modulation system.
The M.sup.2 code modulation system has a variety of excellent characteristic parameters, as discussed hereinbelow.
If the period of a train of modulated signals is expressed as T, the minimum length between transistions T.sub.MIN, which is the minimum distance between a point of signal inversion or transition and the next point of transition, is given by T.sub.MIN =1.0 T for the M.sup.2 code.
The maximum length between transitions T.sub.MAX, that is the maximum distance between a point of signal inversion or transition and the next point of transition, is given by T.sub.MAX =3.0 T.
The window margin T.sub.W, which is a measure of the tolerance of the amount of variation in the signal time axis that may be tolerated with respect to code error, is given by T.sub.W =0.5 T.
The most significant feature of the M.sup.2 modulation code system resides in that it is free of d.c. components, i.e. it is a d.c. free code system. A digital sum value (DSV), used in a known manner in evaluating the content of the d.c. components of modulated signals, gives a measure of the d.c. components by a value of the sum total of scores +1 and -1, with the score +1 and the score -1 being given to a high level "1" and a low level "0" of a code data waveform, respectively. With the M.sup.2 modulation code, occurrences of level transitions are controlled so that the DSV value is within .+-.3, thus making the code d.c. free. The M.sup.2 code modulation, being d.c. free, significantly aids system implementation.
The minimum length between transitions T.sub.MIN is as low as 1.0 T to provide for possibility of high recording density. Data recording or the like in accordance with this format enables high quality data transmission or the like.
Meanwhile, there is a possibility of a still higher recording density in data recording. That is, it may be expected that recording will be made in future using a shorter wavelength and a narrower track.
Considering that tracking accuracy may not meet a predetermined tolerance, if a narrower track width is used, data may be read using a reproducing magnetic head having a wider width than the track width. In such a case, the reproducing magnetic head reads out data recorded on adjacent tracks. That is, crosstalk occurs between adjacent tracks causing deterioration in S/N ratio.
Crosstalk between adjacent tracks becomes most acute with recording in a wavelength region for which an azimuth loss effect of the magnetic head is diminished, that is, with recording at a lower frequency.